The invention relates to a method for cross-linking polymers via a free radical process and for stabilizing such polymers against oxidative and/or thermal decomposition.
Polymers which can be cross-linked via a free radical process such as polyolefins, mixtures with a polyolefin base or with polyolefin as a substantial component, co-polymerizates of ethylene with .alpha.-olefins and ethylene derivative such as vinyl acetate, acrylic esters and styrene, find ever increasing application in many fields of engineering because of their excellent electrical and methanical properties. Of increasing importance is the use of cross-linked polyolefins as insulating material for cables and wires, since such cables and wires can withstand in continuous operation a maximum conductor temperature of 90.degree. C. and during an overload a maximum temperature of 130.degree. C. as a result of the improved thermomechanical properties of the cross-linked polyolefins.
For the radically cross-linked polymers to be suitable for use over a long period of time, a prerequisite is good chemical stability under the conditions of use. However, the above-mentioned polymers, especially if they are cross-linked are subject to accelerated oxidative and thermal decomposition at elevated temperatures. The degradation of their properties substantially shortened the useful life of the polymers. In order to maintain the required characteristics over a substantially longer period of time, such materials must be stabilized against thermo-oxidative decomposition. This is generally accomplished by the use of oxidation inhibitors. Oxidation inhibitors are phenolic or aminic organic compounds which act as radical trapping agents and can therefore interrupt or even prevent the radical decomposition mechanism in the oxidation of organic materials
Information on the most important chemical and structural features of oxidation inhibitors and their operation can be found, for example, in W. L. Hawkins, Polymer Stabilization, Wiley-Interscience, John Wiley & Sons, Inc., New York, London, 1972 and G. K. Cowell, "Additives for Plastic-Antioxidants", Plastic Engineering, pages 51 to 57, (1976).
On the basis of the extensively known material on oxidation inhibitors and the multitude of such products, oxidation inhibitors can be characterized by the following features:
1. They contain at least one phenolic, if possible, sterically hindered hydroxyl group and/or at least one secondary aromatic amino group with a hydrogen which will be easily split off to form a radical, or they are heterocyclic compounds with NH groups. These compounds by transferring at least one hydrogen atom react with the radicals formed in oxidative aging.
2. Due to their chemical structure they are capable of developing low-energy radicals after losing hydrogen or to change into stable, non-radical compounds through secondary reactions.
The effectiveness of the oxidation inhibitors can be influenced by the lattice structure of the aromatic and heterocyclic base or by substituent groups. Further, it is substantially influenced by the solubility and the degree of homogeneity of the oxidation inhibitor in the polymer material.
Oxidation inhibitors are used, if only for cost reasons, only in small concentrations (about 0.01% to 5% by weight). The solubility of the oxidation inhibitors which are strongly polar in polymers which can be radically cross-linked, especially in partially crystalline polyolefins such a polyethylene and polypropylene, is so small, however, that the concentration and molecular-dispersive distribution required for optimum effectiveness cannot be obtained. Because of their low solubility, oxidation inhibitors tend to migrate to the surface of the polymer, from where they would evaporate more or less easily, depending on their vapor pressure. Thus, the polymer is very quickly depleted of these stabilizers, especially at elevated temperatures. As a result, rapid oxidative aging occurs due to the premature loss of the oxidation inhibitors, even if chemical highly effective oxidation inhibitors are used.
To avoid the loss of stability connected therewith, more and more attempts are being made to improve the solubility of the oxidation inhibitors. This is accomplished, for example, via substitution with long-chain alkyl radicals, addition of high-boiling liquid organosilicon compounds as dissolution agents or substitution by silyl radicals (R. V. Albarino and H. Schonhorn, J.Appl.Polym.Sci., 17 3323 (1973); 18, 635 (1974); 19 2667 (1975)). The substitution by silyl radicals is accomplished herein, for example, through reaction of suitable organosilicon compounds with the inhibitors via additional hydroxyl groups which do not participate in the oxidation-inhibiting effect. The hydrogen of these --OH groups is substituted with silyl groups. The --OH groups which function as oxidation inhibitors do not participate in these reactions. Rather, their reaction is expressly avoided, since it is assumed that the oxidation inhibitors may lose their effectiveness thereby.
The process of cross-linking radically cross-linkable polymers is closely related to the process of stabilizing the cross-linked polymers. It is technically advantageous and customary to add stabilizers, including oxidation inhibitors, to the polymers prior to processing them into the formed product with the cross-linking to be carried out thereafter. The cross-linking is therefore carried out in the presence of oxidation inhibitors. The oxidation inhibitors however react in the above-mentioned manner typically with the radicals active in cross-linking, so that cross-linking in the polymer is reduced and at the same time a large part of the oxidation inhibitor is deactivated.
In addition, effective long-term stabilization of the cross-linked polymers is made more difficult by the fact that they are more oxidation-sensitive than non cross-linked polymers. Specifically, long-life radicals remain from the cross-linking reaction which function as initiating radicals for thermo-oxidative aging, and an increased concentration of readily oxidizable groups. In addition, usage of the cross-linked polymers at elevated temperature results in a exponential acceleration of the decomposition reactions. Thus the oxidation inhibitors must meet more stringent requirements though their effectiveness is reduced by the cross-linking reaction.
To sufficiently stabilize the cross-linked polymers, especially polyolefins, relatively high concentrations of oxidation inhibitor are therefore required. High concentrations of oxidation inhibitor, however, cause a heavy reduction of cross-linking in the polymer, so that the concentration of the cross-linking initiator, such as, peroxide concentration or, the radiation dose where cross-linking is by radiation, also must be increased. It is extremely important that the concentrations of these components which reduce each other's effect, during the cross-linking reaction, be accurately matched to each other and to other components. This, however, makes the preparation of the mixture more complicated and expensive.
In spite of the complicated composition of the mixture and the additional use of cross-linkage accelerators, it is not always possible, for example, technically, in radiation-initiated cross-linking of cable and wire insulation with a polyolefin base, to obtain sufficient cross-linking for technical use at production rates necessary for economical manufacture, without either damaging the polymer material or to adversely affect its function as an insulator. The radiation doses required for cross-linking must be supplied in short intervals because heat and hydrogen are liberated during the irradiation. The heat cannot be removed at a sufficient speed when the insultation is of a certain thickness. Therefore, bubbles develop. To ensure production without problems, a way must be found to operate with lower radiation doses. This, in turn, assumes that a method can be found which makes it possible to reduce the interaction between the oxidation inhibitors and the reactive radicals during the cross-linking reaction and, in addition, to accelerate the cross-linking process or to increase the cross-linking yield.